Rick Snow, noted researcher of Indian Cents and die varieties, carefully examined the high-leaf and low-leaf varieties of the Wisconsin State Quarter. Although mainstream media stories have reported this as an accident, Snow analyzes the coins under an electron microscope in order to do forensic analysis.
Snow concludes that the leaf was made with the same tool in a similar manner by a Denver Mint insider with access to the dies. Here is a video Snow produced about his analysis:
I am not sure if this is the “definitive answer,” but it is one of the better theories I have seen.